As a kind of field-effect transistor, a thin film transistor in which a channel formation region is formed in a semiconductor layer formed over a substrate having an insulating surface is known. Techniques for using amorphous silicon, microcrystalline silicon, and polycrystalline silicon for semiconductor layers in thin film transistors have been disclosed (see Patent Documents 1 to 5). A typical application of the thin film transistor is a liquid crystal television device in which the thin film transistor has been put to practical use as a switching transistor for a pixel that constitutes a display screen.